Arrangement for flow measurement

ABSTRACT

An apparatus for measuring fluid flow has a measuring device operating in accordance with the Coriolis principle. The measuring device has an oscillating conduit wherein the fluid to be measured flows through the conduit, and a sensor for sensing the movement of the conduit and generating output signals indicative of the movement. The measuring device generates a measurement signal based on the sensor signal indicative of the flow rate of fluid through the conduit. A counting device is coupled to the measuring device for receiving the measurement signal and calculating the flow rate of fluid through the conduit based on the measurement signal. An error-detecting device is coupled to the measuring device and to the counting device for comparing the measurement signal to a threshold value, and if the measurement signal exceeds the threshold value, the error-detecting device interrupts the transmission of the measurement signal to the counting device and transmits a predetermined measurement signal to the counting device.

The invention relates to an arrangement for flow measurement.

Mass flow measuring arrangements are known which operate by means of atube through which the fluid to be measured flows and which is set intoresonant oscillation, the mass flowing through being calculated fromphase shifts within this tube. The measuring inaccuracies of thesearrangements become undesirably great when a heterogeneous two-phasemixture flows through the metering tube. If the measuring arrangement isinstalled in a line through which various fluids are pumped and if theline is in each case purged with a gas between these fluids,heterogeneous zones occur at the beginning and at the end of each fluidmetering in which zones this fluid is mixed together with a part of thepurge gas.

The invention has the object of minimizing the measuring inaccuraciesduring the time in which a heterogenerous two-phase mixture flowsthrough the metering tube.

This object is achieved in accordance with the characterizing clause ofa main claim.

Advantageous developments of the invention are described in thesubclaims.

During the time in which a recognizable false measurement is provided bythe mass flow measuring instrument, the measuring error is to be reducedby analyzing an artificially generated signal instead of the faultymeasurement signal.

In the text which follows, an illustrative embodiment of an arrangementfor mass flow measurement according to the invention is described andexplained with reference to the drawings, in which:

FIG. 1 shows a basic diagram of a system in which the arrangement formass flow measurement is used,

FIG. 2 shows a measurement signal at the starting time of a metering,

FIG. 3 shows the measurement signal from FIG. 2 at the end of themetering,

FIG. 4 shows a basic circuit diagram of the error correction circuitwithin the arrangement for mass flow measurement,

FIG. 5 shows a comparison of the signal output by the measuringinstrument with the signal received by the counter, during the startingtime of a metering and

FIG. 6 shows a comparison as in FIG. 5 but at the end of the samemetering.

In FIG. 1, B identifies a container which can be supplied with variousraw materials R1, R2, R3 via a line L. Between the feed lines for theraw materials and the container B an arrangement for mass flowmeasurement M is provided. Apart from the raw materials,R the containerB can also be supplied with purge gas G via line L. This purging occursafter each metering during which a particular raw material R is pumpedinto the container B. The extent of each metering or purging,respectively, is determined by opening and closing valves V1, V2, V3 forthe raw materials and Vg for the purge gas.

The measuring instrument Mg within the arrangement for mass flowmeasurement M exhibits an output signal in the form of pulses, thefrequency of which changes with the mass flow through the measuringinstrument. In this arrangement, frequency increases with mass.

During the beginning of a metering, initially only the purge gas ispresent in the metering tube of the arrangement for mass flowmeasurement. Subsequently, when the raw material R flows through themetering tube, a heterogeneous two-phase mixture of purge gas and rawmaterial is present for a particular time in the metering tube andduring this time, the pulses emitted by the measuring instrument do notcorrespond to the actual mass flow. This can be clearly seen from FIG.2: at the beginning of the metering, that is to say of the pumpingprocess for the raw material R, the measuring signals have a very highfrequency and thus indicate a flow of a great amount of mass, whereasthe signal later settles at a value which characterizes a lower massflow. At the beginning of the metering, however, the mass flow cannotyet be as great as later since, especially at the beginning, the rawmaterial R does not yet fully flow through the line and the meteringtube because there is still purge gas in these lines.

At the beginning of metering, the high-frequency measurement signalspartly reproduce capacities which cannot even be produced by the pumpused.

FIG. 3 illustrates that a similar phenomenon can be observed at the endof metering. Here, too, the measurement signal simulates a much greatermass than the mass actually flowing through.

It is true that the false measurements at the beginning and the end of ametering are only within the range of seconds, but the smaller thequantity to be metered, the greater the effect of this error, so that aminimization of this error is therefore desirable.

The correction circuit enabling this error minimization to be performedis shown in FIG. 4. In this figure, Mg designates the measuringinstrument within the arrangement for mass flow measurement whichprovides a measurement signal for the counter Z. Since this measurementsignal, as can be seen from FIGS. 2 and 3, consists of pulses, themeasuring instrument output is followed by a frequency comparator Kwhich provides a signal path for the measurement pulses to the counter Zif the frequency of these measurement pulses is within the tolerancerange. The tolerance range can be predefined, for example, by the factthat only frequencies corresponding to a delivery quantity up to themaximum capacity of the pump connected are analyzed.

If a false measurement becomes apparent by the fact that the outputsignals of the measuring instrument exhibit a frequency which is outsidethe delivery capability of the pump connected, a signal syntheticallygenerated by an oscillator O is set to the counter Z instead of theoutput signal of the measuring instrument Mg. The oscillator shown inFIG. 4 exhibits the capability of generating three differentfrequencies. The oscillator switches, by means of pump feedback signalsfrom the raw material pumps P1, P2 and P3 which in each case exhibitdifferent nominal capacities, a switch S in such a manner that itsoutput signal in each case corresponds to the nominal capacity of thepump which happens to be connected. Purely by way of example, it isassumed that a frequency of 90 Hz at the output of the measuringinstrument Mg corresponds to the maximum capacity of the pump. In thecase where the frequency of the measuring instrument output does notexceed 100 Hz, the frequency comparator K does not output an outputsignal. The switching point of 100 Hz is adjustable and can be atdifferent frequencies for different measuring instruments. At the logicgate "AND1", on the one hand, the measuring instrument signal is nowpresent and, on the other hand, the negated output signal of thefrequency comparator so that, therefore, both inputs of this "AND1" gatereceive a signal. As a result, the output signal of this "AND1" gate isalso 1. It is fed to the logic "OR" gate and from there to the counterZ.

At the same time, the oscillator 0 produces a synthetic signal, thefrequency of which corresponds to the frequency of the measuringinstrument output for the nominal capacity of the pump connected. Thissignal generated by the oscillator is supplied to the "AND2" logic gateas first input which is thus equal to 1. From the frequency comparatorK, an output signal equal to 0 is present as second input at the "AND2"logic gate. The output of this logic gate is thus also at O and nosignal is supplied from here to the counter Z.

If the frequency of the measuring instrument output exceeds 100 Hz andthus a distinct false measurement of this instrument is present, thismeasuring instrument signal is present as first input at the "AND1"logic gate so that this input is equal to a logical 1. The frequencycomparator K also produces an output signal which is equal to alogical 1. Since this, however, is negated before the second input ofthe "AND1" gate, a 0 is present there. Thus, no signal passes from thislogic gate via the "OR" gate to the counter Z. Instead, the outputsignal of the frequency comparator K is present at one input of the"AND2" logic gate which is thus equal to logical 1 and at the otherinput of this logic gate the synthesized signal of the oscillator 0 ispresent so that this second input is also equal to logical 1. Thus, asignal is forwarded from this "AND2" logic gate to the "OR" gate andthrough this to the counter Z.

Accordingly, there is no flow measurement for the time in which aheterogeneous two-phase mixture flows through the metering tube withinthe arrangement for mass flow measurement M since this measurement istoo inaccurate. Instead, a synthesized signal is fed to the counter foranalysis.

FIG. 5 shows, on the one hand, the actual measurement signal which isforwarded by the measuring instrument Mg to the comparator and, on theother hand, the signal which is produced after the correction circuitand is fed to the counter Z. In this arrangement, FIG. 5 shows the startof a metering, that is to say the time after which a raw material R hasbeen pumped through the line L up to the arrangement for flowmeasurement M and is there present in the metering tube as aheterogeneous mixture together with the remaining purge gas G. In thisconnection, it can be seen in FIG. 5 that the measuring instrumentinitially emits for this switching-on process an output signal, thefrequency of which is several times higher than the normal frequencycorresponding to the nominal capacity of the pump. As can be seen fromthe comparison illustrated in FIG. 5, the "measurement signal"synthetically generated by the oscillator is supplied to the counter forthis period of unbelievably high measurement values and it is only whenthe output signal of the measuring instrument reaches realistic valuesthat these information items are fed to the counter.

FIG. 6 shows the same conditions in the reverse time order for theconcluding phase of a metering which is followed by a purging processwith the purge gas G.

A measuring error at the beginning of metering despite the correctioncircuit may be due to the fact that the mass flow in the metering tube,predetermined for the counter by the oscillator, does not correspond tothe mass actually flowing through, since the metering tube also stillexhibits residues of the purge gas G. However, the error between thisACTUAL filling of the metering tube and the NOMINAL filling of themetering tube assumed by the oscillator is much smaller than thedifference between the partly several times excessive capacity specifiedby the measuring instrument Mg and the actual capacity. Referring toFIG. 1, it becomes clear why the correction circuit for the mass flowmeasurement essentially only needs to come into action during thestarting phase of a metering, whereas it is not necessarily needed atthe end phase of this metering. Thus valves V1, V2 or V3 are closed at atime at which the arrangement for mass flow measurement M does not yetindicate the value of raw materials R1, R2 or R3 which is to pass intothe container B. This is because this apparently premature closing ofthe valves V1 to V3 does not yet mean the end of metering of therespective raw material since the quantity of raw material locatedbetween the valve V and the measuring arrangement M in line , L andwhich has not yet been measured, still passes into the container B. Whenthe valve VG is opened and the line L is subsequently purged with thepurge gas G, this residue located in line L is pressed through themeasuring arrangement M into the container B and during this time aheterogeneous two-phase mixture again occurs in the measuring device.The mass located between the measuring arrangement M and the individualvalves V1 to V3 in line L is a predetermined quantity of each of theindividual raw materials R1 to R3 and, therefore, no longer needs to bemeasured by the measuring arrangement M. The second phase, in which themeasuring instrument Mg supplies inaccurate values, therefore, fallsinto a time zone in which no measurement of the flow mass is required.The entire measuring arrangement is therefore not needed at the end ofmetering.

We claim:
 1. An apparatus for measuring fluid flow comprising:ameasuring device operating in accordance with the Coriolis principle,including an oscillating conduit in which the fluid to be measured flowsthrough the conduit, and a sensor for sensing the movement of theconduit and generating output signals indicative of the movement,wherein the measuring device generates a measurement signal based on thesensor signal indicative of the flow rate of fluid through the conduit,said measurement signal being pulses and the frequency of the pulsescorrespond to the flow rate of a fluid through the conduit; a countingdevice coupled to the measuring device for receiving the measurementsignal and calculating the flow rate of fluid through the conduit basedon the measurement signal; and an error-detecting device coupled to themeasuring device and to the counting device for comparing themeasurement signal to a threshold value, and if the measurement signalexceeds the threshold value, interrupting the transmission of themeasurement signal to the counting device and transmitting apredetermined measurement signal to the counting device, wherein theerror-detecting device includes a frequency comparator coupled t themeasuring device for comparing the measurement signal to a thresholdvalue, an oscillator coupled to the counting device for transmittingpredetermined measurement signals to the counting device, and a logiccircuit coupled to the measuring device, the oscillator, and thecounting device for interrupting the transmission of the measurementsignal to the counting device and transmitting a predeterminedmeasurement signal from the oscillator to the counting device upon thefrequency comparator indicating that the measurement signal exceeds athreshold value.
 2. An apparatus as defined in claim 1, wherein thelogic circuit includes a first AND-gate having a first input coupled tothe measuring device for receiving the measurement signal and a secondinput coupled to the frequency comparator for receiving an invertedsignal, a second AND-gate having a first input coupled to the frequencycomparator for receiving an output signal therefrom and a second inputcoupled to the oscillator for receiving predetermined measurementsignals, and an OR-gate having a first input coupled tot he firstAND-gate and a second input coupled to the second AND-gate and an outputcoupled to the counting device for transmitting the signal of eitherAND-gate to the counting device.
 3. An apparatus as defined in claim 1,wherein the oscillator generates a plurality of predeterminedmeasurement signals, each predetermined measurement signal correspondingto the flow rate of a respective pump for pumping material through theconduit.